Many outdoor surfaces are subject to stain or insult from natural sources such as bird droppings, resins, and insect bodies. The resulting stain often leaves unpleasant marks on the surface deteriorating the appearance of the products. These insults are particularly distracting on automotive surfaces such as body panels and glass where the presence of a stain can be can be difficult to remove and potentially dangerous due to reduction in the driver's field of vision.
Self-cleaning coatings have been proposed as a way to prevent staining on surfaces. Traditional self-cleaning coatings and surfaces are typically based on water rolling or sheeting to carry away inorganic materials. These show some level of effectiveness for removal of inorganic dirt, but are less effective for cleaning stains from biological sources, which consist of various types of organic polymers, fats, oils, and proteins each of which can deeply diffuse into the subsurface of coatings. Prior art approaches aim to reduce the deposition of stains on a surface and facilitate its removal capitalizing on the “lotus-effect” where hydrophobic, oleophobic and super-amphiphobic properties are conferred to the surface by polymeric coatings containing appropriate nanocomposites. An exemplary coating contains fluorine and silicon nanocomposites with good roll off properties and very high water and oil contact angles. When used on rough surfaces like sandblasted glass, nanocoatings may act as a filler to provide stain resistance. A drawback of these “passive” technologies is that they are not optimal for use in high gloss surfaces because the lotus-effect is based on surface roughness.
Photocatalytic coatings are promising for promoting self-cleaning of organic stains. Upon the irradiation of sun light, a photocatalyst such as TiO2 chemically breaks down organic dirt that is then washed away by the water sheet formed on the super hydrophilic surface. As an example, the photocatalyst TiO2 was used to promote active fingerprint decomposition of fingerprint stains in U.S. Pat. Appl. Publ. 2009/104086. A major drawback to this technology is its limitation to use on inorganic surfaces due to the oxidative impairment of the polymer coating by TiO2. Also, this technology is less than optimal for automotive coatings due to a compatibility issue: TiO2 not only decomposes dirt, but also oxidizes polymer resins in the paint.
Enzyme containing coatings may specifically target biological stain material such as insect stains or bird droppings. Polymer based enzyme containing coatings, however, are subject to polymer degradation by weathering that the prior art attributes to photolysis, photooxidation, or other scission producing chemical reactions in the structure of the polymeric material itself. The result of environmental insults produces alteration in color, chalking, separation of coating layers, cracking, or reduced gloss. These reactions typically occur on long time scales of continued use in harsh environments.
Therefore, there is a need for new materials or coatings that can actively promote the long term active removal of organic stains on surfaces or in coatings and minimize the requirement for maintenance cleaning.